For a CIS type scanner, the assembly of a contact image sensor and a carrying base can be defined as a scanning module. Conventionally, the contact image sensor is fixed to the carrying base, and a top of the contact image sensor is engaged to an underside of a glass window. The carrying base is movable along a guide rod so as to scan and receive images during the movement of the scanning module.
U.S. Pat. No. 5,801,851 discloses an improvement of a CIS type scanning module and the contact image sensor is installed in a nest and a spring is biased between the contact image sensor and the nest. The spring urges the contact image sensor against an underside of a glass window.
In volume perspective, the nest in the above U.S. patent can receive the contact image sensor so that the volume of the contact image sensor and the nest occupy a large space.
FIG. 1 shows a Taiwanese Publication Patent No. 470276 which discloses a mechanism that maintains a fixed gap between the contact image sensor and the underside of the glass window. A carrying base 11 has an underside 12 which has two sleeves 13 for engaging with a guide rod (not shown). Two support portions 14 extend from two sides of the underside 12 and each of which has a flange 15 which are higher than the contact image sensor 16. A pad 17 is located on a top of each of the flanges 15.
In design perspective, because the flanges 15 of the carrying base 11 contacts the underside of the glass window 18, a downward force is applied on the carrying base 11. So, when every part of the carrying base 11 is made as a one-piece member by the same material, and the contact image sensor 16 is fixed to the carrying base 11, the flanges 15 and the carrying base 11 have complicated deformation under force and the deformation is difficult to be calculated and predicted. For the designers, they cannot calculate and choose correct features of the material to be used so that a try-and-error method is used to choose the factors. However, the method does not meet the requirements of design.
In the transmitting perspective, because the gap has to be maintained between the carrying base 11 and the glass window 18, the contact image sensor 16 has to be fixed on the carrying base 11 such that the elasticity efficient of the carrying base 11 becomes large. Therefore, when the carrying base 11 deforms slightly, a huge bounce force is generated and pushes the carrying base 11 to firmly contact the glass window 18. This increases the friction between the glass window 18 and the carrying base 11 and a larger transmit force is required. This will waste too much energy and the carrying base 11 is difficult to move.
In the perspective of getting material, the flanges 15 need a material that has low elasticity efficient so that it is easily to be deformed and the pads 17 should be made by the material that has low friction efficient so that the pads 17 may move smoothly. However, in practical situation, the pads 17 and the flanges 15 are made by the same material and there is no proper material with low price existed.
In the perspective of the whole volume, the length of the carrying base 11 is roughly the same as that of the contact image sensor 16, so that both of which occupy a large space which does not meet the miniaturized requirement of the trend of design.
Besides, if the contact image sensor 16 is not fixed on the carrying base 11, or it is positioned by force-fitted arrangement, after the carrying base 11 is deformed, it cannot firmly contact the contact image sensor 16. The contact image sensor 16 will move downward and away from the glass window 18. In this situation, the gap between the glass window 18 and the contact image sensor 16 cannot be fixed and results in poor scanning feature.